1. Field of the Invention
This invention relates to radiopaque glasses that are useful as fillers for dental composites.
2. Related Art
Dental restoratives are well known in the art. It is also well known in the art that radiopacity is an important property for a restorative to exhibit. The primary reason for imparting radiopacity to dental restoratives is to facilitate the radiographic diagnosis of dental caries (S. Brent Dove, DDS, MS, “Radiographic Diagnosis of Dental Caries”, available on the web at http://www.nidr.nih.gov/news/CONSENSUS/Brent_DoveA.pdf, Retrieved 12 Apr. 2004). Dental caries formation is accompanied by demineralization of enamel or dentin. Caries are diagnosed by examining dental x-rays for evidence of calcium removal. In order to accurately carry out the diagnosis, the surrounding tissue and dental restoratives must be reasonably radiopaque to provide contrast with demineralized, carious areas. Enamel has a reasonable level of radiopacity due to the presence of calcium in the hydroxylapatite that is a component of both enamel and dentin. When restoratives are placed in dentin or enamel they must be at least as radiopaque as the surrounding healthy hard tissue or the restored areas could be mistakenly identified as carious tissue. Certain well known restoratives such as amalgam zinc phosphate cement, zinc oxide eugenol cement, gold foil or castings as well as other metallic restoratives present no problems regarding radiopacity.
Composite restoratives offer an alternative to these older restoratives. Composites contain two primary components, a resin phase and a filler phase. Composites can be handled easily by the dentist, are usually well tolerated without any irritation by the patient, ensure an aesthetically attractive appearance of the filling, and offer the possibility to move away from amalgam filling materials which have been criticized for physiological reasons. However, the radiolucence of composites can be a big problem, since the resin portion of the composite is not normally radiopaque and the fillers that reinforce the resin may not be either.
Several efforts have been made to impart radiopacity to composite restoratives. They all generally involve trying to incorporate elements with high atomic numbers in either the filler or the resin phase. Chadwick teaches a family of oxyfluoride glasses that contain strontium and/or barium to impart radiopacity, U.S. Pat. No. 5,360,770. Kuhlmann teaches using radiopaque monomers to form a radiopaque resin, U.S. Pat. No. 4,696,955. Dietz teaches the use of barium oxide aluminosilicate glass, U.S. Pat. No. 3,911,581. Michl, et al. describe various prior art compositions including the use of barium sulfate to provide a radiopaque filler, U.S. Pat. No. 4,629,746, col. 1, lines 12–62. Schaefer describes a radiopaque filler for dental composites that incorporates zeolites containing one or more of the elements calcium, strontium, barium, lanthanum, rare earth elements with atomic numbers 58–71, tantalum and hafnium, U.S. Pat. No. 4,375,967. Gasser, et al. describe the use of calcium, strontium and barium salts of hexafluorotitanate, -zironate and -hafnate (TiF6, ZrF6 and HfF6) as well as yttrium fluoride to impart radiopacity to the filler, U.S. Pat. No. 4,767,798. Various compounds containing rare earths have also been proposed as composite fillers for imparting radiopacity. Michl, et al. teach the use of rare earth metal fluorides in amounts of 1% to 50% (elements with atomic numbers from 57–71) as radiopaquing fillers in dental composites, U.S. Pat. No. 4,629,746. They indicate that the rare earth fluorides are particularly well suited for this task because they are more translucent than other rare earth compounds. Cohen, et al. teach dental cement compositions containing lanthanide compounds (oxide, carbonate, nitrate or chloride are used in the examples) in the amount of 1–20%. They state that the lanthanide compounds “ . . . substantially increase the strength of the composition” and they also added fluorides (ytterbium fluoride among them) in amounts up to 6.0%. Although increasing composite strength was the primary stated reason for adding these compounds, another likely unstated reason was to impart radiopacity, U.S. Pat. No. 5,204,398.
Many of these prior approaches involved using a single substance or a small group of substances, to impart radiopacity. Very little choice was available as far as visual translucency was concerned and the use of elements with atomic numbers less than that of ytterbium (70) was not optimal for imparting radiopacity. Moreover, some of the radiopacifiers, such as the brominated monomers of U.S. Pat. No. 4,696,955 may not be safe.
Accordingly, it would be desirable to provide a composite dental restorative composition having a high degree of radiopacity whose translucency could be varied depending on the desired dental application.